A transfer switch is used to switch the source of electric power from a primary source, such as a utility, to a secondary source, such as a generator. Transferring power from the primary source to the secondary source is necessary when the utility experiences a blackout. The transfer switch is also used to switch the power source back to normal utility power when the power outage is over.
A typical transfer switch is composed of an actuating mechanism and a switch stack. The actuating mechanism provides energy to the switch stack to maneuver movable contacts relative to stationary power input contacts. The actuating mechanism operates by storing energy in powerful springs until a control directs the actuating mechanism to release energy from the springs. The released energy rotates a crossbar that runs through the switch stack. There are cams mounted on the crossbar that ride against and drive the movable contacts within the switch stack.
The switch stack is composed of adjacent cassettes. Each cassette, or group of cassettes, carries one-phase of current and includes at least one of the cams that are mounted on the crossbar. The cams within each cassette maneuver at least one movable contact relative to different sets of stationary contacts. The movable contacts engage one set of stationary contacts when power is supplied by the primary source and engage another set of contacts when power is supplied from the secondary source.
Each cassette, or group of cassettes, typically includes a conductive path that conducts one phase of the current through the transfer switch. As the current travels along the path, the conductors along the path generate electromagnetic forces that compress the moving contacts against the stationary contacts. This electromagnetic force counteracts a blow-off force that is generated at the interface between the contacts when there is a current surge.
The individual phases in a three-phase current are not in phase with one another. Therefore, the electromagnetic fields produced by each phase at least partially oppose the fields generated by the other phases. Since the cassettes within a switch stack are typically positioned in close proximity to one another, there are unwanted magnetic interactions between the conductors that reduce the beneficial compressive force that could otherwise be generated by each of the conductors. These magnetic interactions are especially problematic during a current surge, such as current surges generated by short circuits.
The contacts and current paths in transfer switches with high short-circuit withstand capability are typically more massive. The larger size of the contacts and current paths generate even larger magnetic fields such that the magnetic interaction between the current phases is even more problematic in such devices.